The structures and methods disclosed herein relate to photodetectors and, more particularly, to a semiconductor structure comprising a photodetector on stacked trench isolation regions that prevent optical signal loss through a semiconductor substrate below and a method of forming the semiconductor structure.
Generally, in optoelectronics and, particularly, in optoelectronic integrated circuits, on-chip photodetectors (also referred to herein as photosensors or optical receivers) capture optical signals from other on-chip optical devices, such as optical waveguides, and convert the optical signals into electronic signals. In such optoelectronic integrated circuits, oftentimes silicon is used as the core material for optical waveguides because it is transparent to optical signals in the infrared wavelength bands and germanium is used for photodetectors because it absorbs optical signals in those same infrared wavelength bands. While photodetectors are useful in capturing optical signals from on-chip optical devices, they typically are not used to capture optical signals directly from off-chip optical devices, such as optical fibers, because of optical signal loss into the semiconductor substrate at the edge of a chip where direct coupling with the off-chip optical would occur. This is the case when a photodetector is formed on a bulk semiconductor wafer as well as when a photodetector is formed on a relatively thin insulator layer of a semiconductor-on-insulator (SOI) wafer.